Katy Salamati

Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities

This report is intended to provide practitioners with useful information related to establishing safe crossings at roundabouts and channelized turn lanes for pedestrians with vision disabilities. The specific focus areas of the report provide guidance on: identifying under what conditions pedestrians with vision disabilities may experience problems with crossing performance; tying treatment solutions to specific crossing challenges faced by the visually impaired pedestrian population; conducting pedestrian/vehicle studies that help identify performance problems and appropriate treatment strategies; quantifying pedestrian accessibility at a particular crossing; presenting findings from selective field studies performed through this research; developing approaches for extending research findings to other locations; and discussing implications for the practitioner in terms of treatment selection and facility design. The results of this research will be useful to engineers, the accessibility community, policy makers, and the general public to aid in understanding the specific challenges experienced at these facilities by pedestrians with vision disabilities. It is only through the understanding of the components of the crossing task and the particular challenges involved that solutions can be developed, installed, and evaluated appropriately.

Simplified Method for Comparing Emissions in Roundabouts and at Signalized Intersections

An empirically based macroscopic method is presented. It estimates and compares the pollutant emissions generated from signalized intersections and roundabouts. This method was built on a large sample size of real-world second-by-second vehicle trajectories, traffic volumes, and other traffic characteristics collected at signalized intersections and roundabouts in six U.S. states. The basis for predicting and estimating pollutant emissions was the concept of vehicle-specific power. The method enables inclusion of emissions standards and vehicle classes, such as Tier 1 (T1) and Tier 2 (T2) passenger cars (PCs) and passenger trucks (PTs). More than 1,980 vehicle trajectories were analyzed. Traffic variables including intersection capacity, demand-to-capacity ratio (d/c), cycle length, green-to-cycle length ratio, signal progression (i.e., arrival type), and number of lanes were included in the model for analysis and comparison between signals and roundabouts. Application of the method to a case study showed that on average under low d/c (<0.7), roundabouts generated lower emission rates than signalized intersections. As demand approached capacity under high traffic volumes, signalized intersections with favorable progression (i.e., most demand arrived during green phase) generally produced lower emission rates than roundabouts. Signalized intersections with poor progression (i.e., most demand arrived during red phase) generated more emissions than roundabouts. Results also showed that during oversaturation periods (when d/c > 1), the amount of produced emissions increased steadily in roundabouts but increased a large amount at signals.

Potential for metering to help roundabouts manage peak period demands in the United States

Roundabouts generally provide safety and other advantages. During peak hours, however, even moderate demands on an upstream approach can result in long delays and driver frustration over downstream movements. A metering signal is one way to ensure that all demands at a roundabout are adequately served. A roundabout metering signal regulates flow into the circle from one approach and thereby creates larger gaps in the circle for downstream entrants. Although metering signals have been used successfully around the world, little guidance is available for U.S. designers. The goal of this research was to provide U.S. designers with some guidance on the use of metering signals. The authors developed a simple macroscopic model based on the Highway Capacity Manual, validated it by using a simulation model, and performed exercises with it on a number of demand combinations to see where a meter might help. The macroscopic model should be helpful as a quick screening tool. The results from application of the model provide evidence that a meter may reduce delays compared with those seen in an unmetered roundabout with some demands. Although signalized intersections produced lower delays than did metered or unmetered roundabouts in most cases tested, roundabouts were sometimes better. Analysts should consider more than just peak period delays in deciding on the optimum traffic control at an intersection. If, with the aid of a metering signal for a few peak hours, a roundabout produces slightly higher delay levels during those peak hours than a standard traffic signal, with consideration of its many other benefits, a roundabout could well be the optimum design.

The effect of a roundabout corridor's design on selecting the optimal crosswalk location: A multi-objective impact analysis

ABSTRACT Crosswalks located at mid-block segment between roundabouts can provide a good balance among delay, carbon dioxide (CO2) emissions, and relative difference between vehicles and pedestrians speed. However, when considering local pollutant criteria, the optimal crosswalk location may be different to that obtained for CO2. This paper described a multi-objective analysis of pedestrian crosswalk locations, with the objectives of minimizing delay, emissions, and relative difference between vehicles and pedestrians speed. Accounting for the difference between global (e.g., CO2) and local pollutants (monoxide carbon, nitrogen oxides, and hydrocarbons) was one the main considerations of this work. Vehicle activity along with traffic and pedestrian flows data at six roundabout corridors in Portugal, one in Spain, and one in the United States were collected and extracted. A simulation environment using VISSIM, Vehicle Specific Power, and Surrogate Safety Assessment Model models was used to evaluate traffic operations along the sites. The Fast Non-Dominated Sorting Genetic Algorithm (NSGA-II) was implemented to further search optimal crosswalk locations. The results yielded improvements to both delay and emissions by using site-optimized crosswalks. The findings also revealed that the spacing between intersections widely influenced the optimal crosswalk location along a mid-block section. If the spacing is low (<100 m), the crosswalk location will be approximately in 20%–30% of the spacing length. For spacing values between 140 and 200 m, crosswalks would be located at the midway position. When a specific pollutant criterion was considered, no significant differences were observed among optimal crosswalk data sets.

Probit-Based Pedestrian Gap Acceptance Model for Midblock Crossing Locations

Pedestrian gap acceptance has not been explored to the same degree as vehicle gap acceptance. Although the two are similar in concept, there are a variety of pedestrian characteristics and caveats in the interaction between the pedestrian and vehicle modes that require separate pedestrian gap acceptance models. The objective of this research was to analyze empirical observations collected at 27 sites in Alabama, Florida, and North Carolina and to develop pedestrian gap acceptance models at midblock crossings. Goodness-of-fit tests showed that a probit-based, single-lane gap acceptance model, drawn from a noncontrolled pedestrian crossing data set, yielded the best results (max-rescaled R2 = .69). This model involved only two parameters: the size of the gap length in seconds and a binary variable that distinguished between gaps and lag events (first-arriving vehicle, without a prior lead vehicle, to open the gap). An increase in gap length was associated with an increased probability that a pedestrian would cross, whereas a lag event had a negative coefficient, which meant that a pedestrian was less likely to accept a lag than a gap, given the same length in seconds. Other variables that described crosswalk characteristics as well as pedestrian and driver behavior did not emerge as significant factors in the model. The research reported in this paper offers a new, robust pedestrian gap acceptance model that can be used in traffic operational analysis. The model can be incorporated into microsimulation packages and thus improve the accuracy of pedestrian behavior modeling at midblock crossings in the future.

Pedestrian and bicyclist accommodations and crossings on superstreets

This research considered the unique challenges for pedestrians and bicyclists at superstreet intersections and recommends crossing alternatives for both users. For pedestrians, the options included the diagonal cross, median cross, two-stage Barnes Dance, and midblock cross. For bicyclists, the options included the bicycle U-turn, bicycle use of the vehicle U-turn, bicycle direct cross, and midblock cross. These options were analyzed through microsimulation on the basis of average stopped delay, average number of stops, and average travel time per route. Various parameters were analyzed for each crossing configuration, including two signal cycle lengths (90 and 180 s), two signal splits (60/40 and 75/25), two signal offset designs (vehicle platoons arrive simultaneously and at various offset times), and two midblock distances (600 and 800 ft). The recommended pedestrian crossing was a combination of the diagonal cross with the midblock cross. The parameters that decreased travel time for pedestrians were a 90-s cycle length, a 60/40 signal split, and an offset signal design in which the vehicle platoons arrived at different times. For bicyclists, the bicycle direct cross resulted in the lowest average number of stops and the lowest average travel time, whereas the vehicle U-turn showed the lowest stopped delay. The parameters that decreased travel time for bicyclists included a 90-s cycle length and the vehicle platoons arriving at different times. The recommended options for bicyclists at a superstreet were a combination of the bicycle direct cross and the midblock cross.